Why Don't we have a Lunar Eclipse Every Month?

On the night of May 15/16 we will be able to see a total lunar eclipse. Lunar eclipses can only happen when the Moon is full and it passes behind the Earth and into its' shadow. But that brings to mind a question. It would make sense to think that because we have a full moon every month that we should also therefore have a lunar eclipse every month.

If the Moon's orbit around the Earth were in the same plane as the Earth's around the Sun (the ecliptic,) we would indeed have a monthly eclipse. However, the Moon's orbit is inclined about 5 degrees to the Earth's orbit. The Moon passes through the ecliptic only twice a month at a pair of points called the nodes. The rest of the time the Moon is either above or below the plane of the Earth's orbit and does not pass directly through the Earth's shadow.

The Moon is not only acted on by its' gravitational bond with the Earth, the Sun also plays a part by trying to flatten out the Moon's orbit so that it is on the ecliptic. This makes the Moon "wobble" on its' axis just like the Earth does. The effect is like that of a spinning top. Because of gravitational interaction with the Sun, the line of nodes of the Moon's orbit moves over time, taking 18.6 years to make a complete cycle. This 18.6 year sequence of eclipses is called the saros cycle and during this cycle solar and lunar eclipses occur about every six months. The exact dates of the eclipses change and do not repeat for that 18.6 year cycle.

The change in the line of lunar nodes does more than just affect when eclipses occur. It also affects changes in the northern and southern extremes of the Moon as we view it by plus or minus five degrees. The farthest north of the celestial equator that the moon can be is 23 1/2 degrees plus the 5 degree inclination of the lunar orbit or 28 1/2 degrees. The farthest south, similarly, is -28 1/2 degrees. As an example, If your celestial equator is about 50 degrees up from the southern horizon, that means the Moon's height can change from as high as 78.5 degrees from the horizon to 21.5 degrees. That produces changes in the length of a moonlit night, making them longer or shorter depending on whether the Moon is at a northern or southern extreme.

It is interesting to note that the saros cycle was noticed by ancient cultures. The ancient Egyptians recorded the saros, but they didn't understand what caused it. They did realize that this wobbling was directly linked to eclipses. It was they who gave the saros its' name, and correctly observed its' period of 18.6 years.